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Tree nodes can have data attached to them. Some of the most comonly used include:

Bloom Filter

Cardinality Estimator

Secondary Counts (think views and bytes for URLs)

Creation or touch time (for TTLs)

Top-k Estimator

Histograms

Data attachments can be purly informational (such as a sum) or affect the rest of a tree. A limit.top limits how many children a node can have. Continuing our product code example. For each version of a product we might want to keep track of how many domains that product is used for, and a small sample of the most popular domains, but not store every single domain. Cardinality and top-k estimators allow you to do that: